$f(x, y) = (5y + 9x, yx + 1)$ $\text{curl}(f) = $
The formula for curl in two dimensions is $\text{curl}(f) = \dfrac{\partial Q}{\partial x} - \dfrac{\partial P}{\partial y}$, where $P$ is the $x$ -component of $f$ and $Q$ is the $y$ -component. Let's differentiate! $\begin{aligned} \dfrac{\partial Q}{\partial x} &= \dfrac{\partial}{\partial x} \left[ yx + 1 \right] \\ \\ &= y \\ \\ \dfrac{\partial P}{\partial y} &= \dfrac{\partial}{\partial y} \left[ 5y + 9x \right] \\ \\ &= 5 \end{aligned}$ Therefore: $\begin{aligned} \text{curl}(f) &= y - 5 \end{aligned}$